The present invention relates to metal powders and electrolytic capacitors using the metal powders as well as methods of making the powders and electrolytic capacitors.
For many years, it has been the goal of various researchers to develop niobium electrolytic capacitors because of the high di-electric constant of its oxide and the relatively low cost of niobium compared to a variety of other metals. Initially, researchers in this field considered the possibility of using niobium as a substitute for tantalum capacitors. Accordingly, many studies were conducted to determine the suitability of replacing tantalum with niobium.
In some of these studies, however, it was concluded that niobium has serious fundamental deficiencies that needed to be resolved, thus inferring that niobium was not an acceptable substitute for tantalum. (See J. Electrochem. Soc. p. 408 C, December 1977). In another study, one conclusion reached was that the use of niobium in solid electrolytic capacitors seems very unlikely due to various physical and mechanical problems, such as field crystallization. (Electrocomponent Science and Technology, Vol. 1, pp. 27–37 (1974)). Further, in another study, the researchers concluded that anodically formed passive films on niobium were different from electrical properties accomplished with tantalum and that the use of niobium led to complexities which were not present with tantalum. (See Elecrochimica Act., Vol. 40, no. 16, pp. 2623–26 (1995)). Thus, while there was initial hope that niobium might be a suitable replacement for tantalum, the evidence showed that niobium was not capable of replacing tantalum in the electrolytic capacitor market.
Besides tantalum electrolytic capacitors, there is a market for aluminum electrolytic capacitors. However, the aluminum electrolytic capacitors have dramatically different performance characteristics from tantalum electrolytic capacitors.
A driving force in electronic circuitry today is the increasing move toward lower Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). As IC performance increases with submicron geometry, there is a need for lower power supply voltage and noise margin. At the same time, increasing IC speeds require higher power needs. These conflicting requirements create a demand for better power management. This is being accomplished through distributed power supplies which need larger currents for decoupling noise. Increasing IC speeds also mean lower switching times and higher current transients. The electrical circuit must, therefore, also be designed to reduce the transient load response. This broad range of requirements can be met if the circuit has large enough capacitance but low ESR and ESL.
Aluminum capacitors typically provide the largest capacitance of all capacitor types. ESR decreases with increase in capacitance. Therefore, currently a large bank of high capacitance aluminum capacitors are used to meet the above requirements. However, aluminum capacitors do not really satisfy the designers' requirements of low ESR and ESL. Their mechanical construction with liquid electrolyte inherently produce ESR in the 100s of milliohm along with high impedance.
In the past, solvents such as ethanol have been used during a milling process to make metal flakes. The solvent is desirable to provide wet milling of metal to form flakes. It has been discovered that the use of alcohols and other carbon and oxygen containing solvents for wet-milling or wet-grinding of metal powders to form flakes can create a number of problems. In particular, oxygen present in wet-milling solvents can be released from the solvent due to the ease with which C—O bonds (carbon to oxygen bonds) are broken. As oxygen enters the system from the broken solvent molecules, the oxygen can react or be present with the metal flakes being formed or with stainless steel milling medium and cause impurities in the resulting flakes. It is believed that breakage of C—O bonds in wet-milling solvents leads to higher levels of carbon and iron contamination in the produced flakes and causes a resultant corrosive environment.
It is desirable to provide a wet-milling solvent which is substantially inert with respect to metal flakes being formed by the wet-milling process and with respect to the milling medium. It is also desirable to provide a wet-milling solvent which does not break down or decompose during wet-milling.